SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 930–935 (1999)

Availability of Layered Certified ReferenceMaterials for Industrial Application of GlowDischarge Spectrometric Depth Profiling

Michael Winchester, R.1* and Uwe Beck1 Analytical Chemistry Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology(NIST), Gaithersburg, MD 20899, USA2 Bundesanstalt fur Materialforschung und-Prufung (BAM), FG VIII.2, Unter den Eichen 87, 12205 Berlin, Germany

Owing primarily to speed, low cost and ease of use relative to competing surface analysis techniques, glowdischarge spectrometry (GDS) has become an economically important depth profiling method for certainniche industries in Europe and Japan. For the industrial application of this method, layered certified referencematerials (CRMs) are needed to ensure accuracy, as well as comparability between individual users. Asurvey of existing CRMs has been conducted to evaluate the availability of layered CRMs with appropriatecharacteristics. No materials with the desirable characteristics necessary for efficient implementation werefound. These results indicate a need for the production of new layered CRMs. The survey was conductedby an advisory group assembled as part of the activities of the Glow Discharge Spectrometry Subcommittee(ISO TC201/SC8) associated with the ISO Technical Committee on Surface Chemical Analysis (ISO TC201).Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: certified reference materials; depth profiling; glow discharge spectrometry; ISO

INTRODUCTION

Characterization of films and coatings is an importantanalytical problem across a wide range of technologies.For example, electronic devices are typically manufac-tured by combining several interfaces between metals,semiconductors and insulators. The performance of thesedevices depends dramatically on the chemical and physi-cal properties of both the materials used and the interfacesfabricated with them. Therefore, it is often necessary toassess both the chemical and physical characteristics of thevarious layers and interfaces. Among other things, charac-terization may include assessment of layer thickness andelemental composition, as well as interface width.

As another example, overlays consisting of organicpolymers (e.g. polyester) are becoming a popular means ofprotecting unpainted metallic coatings on steel. The pro-tection provided by such overlays depends largely on thelayer thickness. Therefore, quality control demands thatthe thickness of these overlays be determined routinely.Further, pigments are sometimes added to the overlays foraesthetics. In such cases, it may be helpful to assess theelemental composition of the coating in order to controlthe pigment/polymer ratio.

Given the widespread technological importance of filmsand coatings, it is fortunate that a number of method-ologies currently exist with which to characterize them.The most prominent examples are AES, XPS, SIMS andRBS. Each of these techniques has its own advantages and

* Correspondence to: M. R. Winchester, Analytical Chemistry Divi-sion, Chemical Science and Technology Laboratory, National Instituteof Standards and Technology (NIST), Gaithersburg, MD 20899, USA.E-mail: mrw@nist.gov

disadvantages. As a result, they tend to be complementaryrather than competitive. Taken as a whole, this set ofmethods provides impressive capabilities, including depthresolution as small as 1 nm, lateral resolution as smallas 10 nm, detection limits as low as 1µg kg�1 and theability to provide chemical bonding information. Unfortu-nately, all of these methods also share several shortcom-ings. These include high initial instrumentation costs (e.g.$US 1 million is not uncommon), high operational costs(e.g. of the order of 5% of the initial investment per year,excluding overhead costs associated with laboratory spaceand personnel) and low sample throughput. Also, accuratequantification of elemental mass fractions and depth scalestends to be problematic for all four techniques.

Over the past two decades, glow discharge spectrometry(GDS)1 – 3 has emerged as an important alternative depthprofiling method for some analytical problems of indus-trial and technological significance. Glow discharge spec-trometric depth profiling combines several characteristicsthat are advantageous compared to the aforementionedmethods. Notably, instrumentation and operational costsare much lower, averaging less than $US 150 000 and$US 10 000 per year, respectively. Also, sample through-put is comparatively fast, with the time required for depthprofiling of even extremely thick layers (e.g. 200µm)typically ranging between a few minutes and a few hours.Further, meaningful quantification of elemental mass frac-tions and depth scales can frequently be accomplished in astraightforward manner, owing largely to the fact that themethod is less matrix dependent than most other methods.This means that GDS often can be used readily to evalu-ate layer thickness and elemental composition, as well asinterface characteristics. Other advantages of GDS depthprofiling are that it is inherently a simultaneous, multi-element method and is capable of detecting all of the

CCC 0142–2421/99/100930–06 $17.50 Received 19 October 1999Copyright 1999 John Wiley & Sons, Ltd. Revised 5 May 1999; Accepted 7 May 1999

LAYERED CRMs FOR GDS DEPTH PROFILING 931

non-metallic elements, including hydrogen. Finally, themethod is applicable to both single- and multilayers, tolayers and/or substrates that are either electrically con-ductive or non-conductive and to either thin (i.e.<1 µm)or thick (i.e.>1 µm) layers with equivalent ease.

Like all other analytical techniques, GDS also haslimitations. These include the fact that depth resolutionin many cases may be poorer than it is for the competingmethods, and the fact that GDS inherently provides nobetter than millimeter-scale lateral resolution. Further, noinformation regarding chemical bonding on the analyzedsurface is attainable. Although the detection sensitivityprovided by GDS depth profiling is usually not quite asgood as that provided by SIMS, it is generally muchbetter than for the electron spectroscopies, and is certainlyacceptable for many applications.

To date, GDS depth profiling has found several nichesin European and Japanese industry. Generally, theseniches are characterized by analytical problems that do notrequire chemical bonding information, lateral resolutionor the ultimate in depth resolution or detection sensitivity,but for which GDS can provide the desired informationat substantial savings of time and money. Most of theapplications of GDS depth profiling in industry to datehave involved the characterization of thick coatings. Thisis because the competing methods are slow and tediouswhen applied to such coatings. Having recognized thepotential economic benefits to industry afforded by GDSdepth profiling, several commercial analytical instrumentcompanies now manufacture GDS depth profiling instru-mentation.

The most important industrial application of GDS depthprofiling to date is the characterization of galvanized coat-ings on steel.4 – 12 Such coatings are used extensively inthe automotive and home appliance industries, becausethey exhibit excellent corrosion resistance. They may alsofind increasing use in the aerospace industry in the future,because Cd coatings are being phased out of use, owingto environmental concerns. The plating material is usu-ally Zn or some Zn alloy, and the coating thickness istypically 5–10µm. Good quality control demands routinedetermination of the coating thickness or coating mass (i.e.mass of coating divided by area), as well as the elementalcomposition of the layer. Additionally, it may be nec-essary sometimes to quantify elemental impurities at theinterface between the layer and substrate. In some cases,phosphate or chromate passivation layers are depositedon the galvanized coating in order to modify the pro-cessing and application properties of the material or toenhance further the corrosion resistance. If this is done,the passivation layer also must be examined. Glow dis-charge spectrometric depth profiling can be used readilyand relatively inexpensively for all of these purposes. Asa result, it has become the predominant method used inEuropean and Japanese industry for depth profiling of gal-vanized coatings.

Another important industrial application of GDS depthprofiling is the characterization of hard coatings.13 – 29

These coatings are commonly applied to materials suchas cutting tools and engine parts to improve wear resis-tance and surface durability. Hard coatings are typicallycomposed of carbides, nitrides, oxides or borides of theGroup IV–Group VI elements. Even though TiN tradi-tionally has been the most important material used toproduce hard coatings, TiC, TiCN, CrN and other nitridic

compounds, as well as WC, also have been applied exten-sively. Hard coatings are usually produced using chemicalvapor deposition (CVD), physical vapor deposition (PVD)or plasma-enhanced CVD (PECVD). These productionmethods are extremely complex and there is a need foranalytical methods that can be used for process control.Glow discharge spectrometric depth profiling is beingused widely for this purpose because characterization ofthe coatings can be performed quickly, straightforwardlyand inexpensively.

There are many other important applications of GDSdepth profiling in industry. These include the examina-tion of oxide scales on steel9,30 – 36 and high-temperaturealloys,35,37,38 evaluation of surface-hardened steels39 andprocess control of Sn coatings used in the food canningindustry. Additionally, the method has shown potential forapplicability in some non-traditional areas, such as exam-ination of polymeric and painted surfaces.40 – 43 Further,GDS has recently begun to find some limited applicationin the semiconductor industry.44

Although GDS depth profiling has been quite usefulin industry to date, one limiting factor has been theapparent unavailability of layered certified reference mate-rials (CRMs) with the characteristics necessary for effec-tive implementation. Such CRMs are needed primarilyas check standards for quality control, in order to under-pin industrial measurements. In the absence of appropriatelayered CRMs, many companies have adopted the prac-tice of periodically checking GDS results with a secondmethod and/or producing in-house reference materials.Although industry has managed to operate in this way,layered CRMs would be helpful for ensuring accuracy, aswell as comparability between individual users.

Recently, the Glow Discharge Spectrometry Subcom-mittee (ISO TC201/SC8) associated with the ISO Tech-nical Committee on Surface Chemical Analysis (ISOTC201) appointed an advisory group to investigate theavailability of layered CRMs with appropriate character-istics for GDS depth profiling. The advisory group wasestablished in the hope of identifying CRMs that could beused by the GDS community, and was predicted to focusattention on the need for such CRMs, thus serving as astimulus for the production of new materials. This advi-sory group, which was in part composed of the authors ofthis paper, has now completed the task. The findings of theadvisory group are discussed in detail herein. Before elu-cidating those findings, however, it is helpful to describebriefly GDS depth profiling.

BRIEF OVERVIEW OF GDS DEPTH PROFILING

Glow discharge spectrometry is based upon the use of aglow discharge device for the production of analytical sig-nals. The typical glow discharge device is a small, metalvacuum chamber filled with an inert gas, usually Ar, toa pressure of 100–1500 Pa. A potential of 500–1500 Vis applied between the sample, which serves as the cath-ode, and the chamber, which serves as the anode. Thegas breaks down to produce free electrons and positiveions. This results in ionic bombardment and sputter ero-sion of the sample surface. Once in the gas phase, atomssputtered from the sample may be excited electronicallyor ionized via collisions with energetic particles. There-fore, the sputtered atoms may be detected and quantified

Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 930–935 (1999)

932 M. R. WINCHESTER AND U. BECK

by means of optical emission spectroscopy (GD-OES) ormass spectrometry (GD-MS). Given that a large major-ity of the sputtered atoms remain in the ground electronicstate, the sputtered material may be quantified also viaatomic absorption spectrometry. Although any of thesemodes conceivably can be utilized for depth profiling,GD-OES is by far the most common and well developedfor this purpose. It is important to note that the desirableCRM characteristics, which are discussed in the follow-ing section, are independent of the analytical mode. Forinformation, some analytical figures of merit for GD-OESand GD-MS depth profiling are given in Table 1.

A typical (i.e. Grimm type45,46) glow discharge deviceused in GD-OES depth profiling is depicted in Fig. 1.As shown, the sample is pressed against an O-ring bymeans of a reamer so that it makes good contact with thecathode plate. In this way, the sample effectively becomesthe cathode. The distance between the sample surface andthe anode is usually¾0.2 mm. The reamer is also usedto clean sputtered material from the inside of the anodebetween analytical burns.

In order to perform depth profiling, the device is opti-cally coupled to a polychromator that allows multipleemission wavelengths to be monitored simultaneously.Beginning with ignition of the discharge, the emissionintensities for all of the analytical wavelengths of interestare recorded as a function of sputtering time. The resultingqualitative depth profile (e.g. Fig. 2(a)) may be trans-formed into a quantitative depth profile (e.g. Fig. 2(b))by converting the emission intensities into elemental massfractions and the sputtering time into sputtered depth. For-tunately, these conversions can be accomplished through

Table 1. Analytical figures of merit for GD-OES and GD-MS depth profiling2

Figure of merit GD-OES GD-MS

Limit of detection .mg kg�1/ 1 100 0.01 10Minimum detectable number ofatoms .cm�2/

1013 1015 1011 1014

Minimum information depth (nm) 1 1Relative depth resolution (%) 5 10 5 10Sputter erosion rate .nm s�1/ 1 100 0.1 10

Figure 1. Diagram of a typical glow discharge device used forGD-OES depth profiling.

Figure 2. (a) Qualitative GD-OES depth profile of a Ti/Al mul-tilayer stack deposited onto an Al alloy substrate. (b) Corre-sponding quantitative depth profile after conversion of emissionintensities into elemental mass fractions and sputtering time intosputtered depth. Discharge conditions, which were not neces-sarily optimum for analysis, were 800 V and 50 mA, operated indc mode. Note that <25 s were required to sputter through theentire stack, which had an overall thickness of ¾2 µm.

any one of several quantification procedures involvingonly bulk CRMs.2,3 Although generally applicable quan-tification procedures for other modes of detection (e.g.GD-MS) are yet to be realized, it is likely that such pro-cedures will also require only bulk CRMs. This being thecase, one may legitimately ask why layered CRMs are ofinterest for GDS depth profiling.

As mentioned in the Introduction, layered CRMs areprimarily needed as check standards for ensuring depthprofiling accuracy and comparability between individualusers. The need for check standards is emphasized in thatthe commonly employed quantification routines incorpo-rate several assumptions (discussed in detail elsewhere2,3)that may affect accuracy. As added benefits, layeredCRMs would also be helpful for making small adjustmentsto calibration, for full evaluation of analytical figures ofmerit (e.g. depth resolution), for determining sputteringrates and for further development of GDS instrumenta-tion and methodology. Finally, for specific applications,

Surf. Interface Anal. 27, 930–935 (1999) Copyright 1999 John Wiley & Sons, Ltd.

LAYERED CRMs FOR GDS DEPTH PROFILING 933

layered CRMs might be employed in alternative calibra-tion schemes. For example, if GD-OES is being usedroutinely to determine the thickness of galvanized coat-ings on steel, then matrix-matched CRMs with certifiedcoating thicknesses might be used to calibrate the timenecessary to sputter through coatings of various thick-nesses at a given set of discharge conditions. Such anapproach would be much simpler to implement than anyof the normal quantification procedures.

THE SURVEY

In order to assess fully the availability of layered CRMsrelevant to GDS depth profiling, the advisory group con-ducted a survey of existing CRMs throughout the world.The advisory group began by establishing a list of desir-able characteristics that existing CRMs should have inorder to be useful for GDS depth profiling. With appro-priate commentary, the characteristics are:

(1) Generally, the layered surface should have a surfacearea of at least several square centimeters, so thatmultiple spots can be examined.

(2) The CRM should be structurally rigid, to enable easymounting onto the glow discharge device and toensure that the material does not fracture or deformsignificantly under the mechanical stress placed uponit.

(3) The layered surface should be flat, in order to obtainadequate vacuum.

(4) The layered surface should be smooth. This is alsoneeded to obtain adequate vacuum. Additionally, thesmoothness of the surface in part determines theobtainable depth resolution.

(5) The layered surface should be laterally uniform, interms of both elemental mass fractions and depthdistributions, over the analyzable area.

(6) At least some of the quantitative analytical informa-tion that is attainable with GDS depth profiling (e.g.coating thickness, coating mass, elemental composi-tion) must be certified. However, the particular infor-mation that must be certified is not specified here,because current applications of GDS depth profilingin industry are broad and because the applicability ofthe method is continually increasing.

(7) The CRM should be stable over long periods of time.

With the established list of desirable characteristics, exist-ing CRMs could be evaluated in terms of their relevanceto GDS depth profiling. The advisory group did not limitits attention to CRMs resembling samples that are cur-rently of industrial importance. Rather, all layered CRMswere considered, including those with either single-layeror multilayer structures and those with either ‘thick’ (i.e.>1 µm) or ‘thin’ (i.e.<1 µm) layers. Furthermore, CRMsincorporating either conductive or insulating layers and/orsubstrates were considered. This approach ensured that theresults of the survey will remain useful in the future as theapplicability of GDS depth profiling continues to increase.

The survey was conducted primarily through the useof the COMAR (COde MAteriaux Reference) CertifiedReference Materials Database. This is an internationaldatabase cataloguing almost 9000 CRMs of various typesfrom both major and minor manufacturers in more than

20 countries. Although the documentation associated withthe database is poor, a very large majority, if not all, ofthe major CRM producers seem to be included. There-fore, the search conducted with the COMAR databaseis thought to be thorough. Additionally, however, thisapproach was supplemented by personally contacting asmany major CRM producers worldwide as possible. Fur-thermore, several minor producers that seemed promisingwere also contacted. Although it is possible that relevantCRMs escaped the attention of the advisory group, this isconsidered to be unlikely.

The survey thus conducted revealed more than 40existing layered CRMs. Unfortunately, none of these haveall of the desirable characteristics to be considered usefulfor GDS depth profiling. The most common shortcomingis that the physical characteristics are inappropriate. Forexample, many existing CRMs are too thin and fragileto be mounted easily onto a glow discharge device. Asanother example, the surface areas of the layered surfacesof some CRMs are so small that only one or two spotscan be examined. Obviously, the reason why the physicalcharacteristics are often unsatisfactory for GDS depthprofiling is that the materials were originally designed andcertified specifically for other measurement methods.

Although it is true that no CRMs with all of thedesirable characteristics could be identified, a number ofCRMs that may be of some limited utility were found.For the benefit of the GDS community, Table 2 containsa list of these CRMs. It is important to note that noneof these materials are similar to samples that are nowcommonly characterized using GDS depth profiling. As aresult, these CRMs are not relevant to the method as itis currently used within industry. However, the relevancyof the materials may change as the applicability of GDSdepth profiling expands.

CONCLUSIONS

The advisory group was unable to find any existing lay-ered CRMs with all of the desirable characteristics neededfor GDS depth profiling. Although some CRMs with lim-ited utility were found, these CRMs are not relevantto current industrial applications of the method. Conse-quently, there is a need for new layered CRMs to bedeveloped. If new CRMs are produced, they should beprepared with specific applications of industrial or tech-nological importance in mind. Given the current uses ofGDS depth profiling in industry, some important candi-date materials include galvanized and hard coatings onsteel, oxide scales on steel and high-temperature alloysand surface-hardened steels, as well as Sn, Ni and Crplatings on various metals. Moreover, new CRMs proba-bly should be designed with physical characteristics thatwould allow them to be used not only for GDS depth pro-filing but also for other surface analysis techniques. In thisway, the new CRMs would provide broad-based benefitsto the surface analytical community.

At this point, it is important to emphasize that theproduction and certification of CRMs is a very labori-ous, time-consuming and expensive endeavour. This isespecially true for layered CRMs, which present spe-cial challenges. At the same time, national metrologicalinstitutes (NMIs), which are endowed with the responsi-bility of producing and certifying CRMs (e.g. NIST and

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934 M. R. WINCHESTER AND U. BECK

Table 2. Certified reference materials with limited utility for GDS depth profiling

Producer Reference material Description Certified parameter(s) Nominal certified value(s)a

UNIIMb 1180 89 Ag on bronze Coating mass .kg m�2/ 0.004 and 0.8504982 89 Si doped with B B depth distribution Not applicable4983 89 Si doped with P P depth distribution Not applicable5441 90 GaAs doped with Si Si depth distribution Not applicable5442 90 GaAs doped with Cr Cr depth distribution Not applicable

IRMMc CRM 328 Au on Ni Coating mass .mg cm�2/ 0.863, 1.570, 5.823 and 9.597CRM 564 SiO2 on Si Layer thickness (nm) 10, 50 and 120

NISTd SRM 1357 Cu on steele Layer thickness (µm) 6, 20 and 48SRM 1358 Cu on steele Layer thickness (µm) 80, 225 and 1000SRM 1359 Cu on steele Layer thickness (µm) 48, 140, 505 and 800SRM 1360 Cu on steele Layer thickness (µm) 2.5, 6, 12 and 20SRM 1361a Cu on steele Layer thickness (µm) 6, 12, 25 and 48SRM 1362a Cu on steele Layer thickness (µm) 40, 80, 140 and 205SRM 1363a Cu on steele Layer thickness (µm) 255, 385, 505 and 635SRM 1364a Cu on steele Layer thickness (µm) 800, 1000, 1525 and 1935SRM 1371 Au on Fe Ni Co alloy Coating mass .mg cm�2/ 1.5

Layer thickness (µm) 0.8SRM 1373 Au on Fe Ni Co alloy Coating mass .mg cm�2/ 6.0

Layer thickness (µm) 3SRM 1374 Au on Fe Ni Co alloy Coating mass .mg cm�2/ 14.0

Layer thickness (µm) 7SRM 2135c Ni/Cr multilayer stack Overall Ni coating mass .µg cm�2/ 226

Overall Cr coating mass .µg cm�2/ 190Individual Ni layer coating mass 56.5.µg cm�2/Individual Cr layer coating mass 38.0.µg cm�2/Individual Ni layer coating mass š3.8uniformity (% relative)Individual Cr layer coating mass š4.0uniformity (% relative)Individual Ni/Cr bilayer coating š3.3mass uniformity (% relative)

SRM 2136 Cr/CrO multilayer Overall Cr coating mass .µg cm�2/ 175.3stack

Individual Cr layer coating mass 22.µg cm�2/

SRM 2137 Si doped with 10B Retained dose of 10B .µg cm�2/ 0.017SRM 2321 Sn Pb alloy on Cu Coating mass .mg cm�2/ 6.8

Sn mass fraction in coating (%) 60

a As given either in the COMAR citation or in the producer’s catalog.b UNIIM, Mr Leonov, Krasnoarmeiskaya St. 4, 620219 Ekaterinburg, GSP-824, Russia; Tel: 343-2-55-26-18; Telex: 721911 PSB-SU;Fax: 343-2-55-20-39.c IRMM, Management of Reference Materials Unit, Retieseweg, 2440 Geel, Belgium; Tel: C32 14 571 722; Fax: C32 14 590 406.d NIST, Standard Reference Materials Program, Building 202, Room 204, Gaithersburg, MD 20899, USA; Tel: 301-975-6776; Telex: TRT197674; Fax: 301-948-3730.e Overcoated with a thin layer of Cr for wear resistance.

BAM), usually have very limited human and financialresources, relative to the needs of industry for CRMs. Asa result, NMIs typically receive many more requests forthe production of various types of CRMs than can pos-sibly be addressed. Consequently, each NMI must care-fully consider many factors in deciding which new CRMsto produce. Some of the factors that must be balancedinclude: the range of application; the monetary signifi-cance, including prediction of economic impact; scientificand technological significance; and the uniqueness of thecapabilities of the NMI. At this point in time, it is unclearhow many NMIs will decide to invest in the productionof new layered CRMs relevant to industrial application ofGDS depth profiling.

None the less, BAM has recently undertaken an inves-tigation of several potential multilayer CRM candidatesintended specifically for GDS depth profiling. These

candidates will be evaluated under a project of the SurfaceChemical Analysis Technical Working Area (TWA 2)of the Versailles Project on Advanced Materials andStandards (VAMAS).47 Multilayer candidates are Ti/Al,SiO2/Si3N4 and SiO2/TiO2, chosen because of their tech-nological and industrial importance. A portion of theplanned work involves GDS depth profiling round-robins,and all GDS practitioners are invited to participate.Inquiries should be directed to the project managerfor the VAMAS activity.48 Certainly, the new layeredCRMs will make a valuable contribution to GDS depthprofiling.

Acknowledgements

The authors would like to thank Han Thai (Standard Reference Data Pro-gram, NIST) for valuable assistance in performing the COMAR database

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LAYERED CRMs FOR GDS DEPTH PROFILING 935

search, as well as Philippe Hunault (LECO Corp., St Joseph, MI, USA)for his help in performing the French part of the search, for translat-ing the French citations into English and for many helpful discussions.The authors are also grateful to Thomas Wirth (BAM) for providingFig. 2 and to Arne Bengtson (SIMR) for several helpful discussions and

suggestions. As mentioned in the Introduction, the authors of this paperwere two of the members of the advisory group that conducted the sur-vey. The contributions of the remaining two members—Joel Mitchell(LECO) and Richard Payling (Surface Analytical, Wollongong, NSW,Australia)—are also acknowledged.

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Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 930–935 (1999)